How to Turn a Macbook Into a Wireless Router

Navigate to Apple>System Preferences> Network

On the Network window click on AirPort from the left Menu.

Make sure your AirPort is turned on. If it isn't press the "Turn AirPort On" button.

Under the "Network Name" Drop down menu choose "Create Network".

Enter your desired SSID and Password and Create.

Your router is up but its not sharing internet yet. Navigate back to System Preferences.

Choose Sharing.

Enable the "Internet Sharing" feature by activating the checkbox next to it in the left menu.

You will receive a message asking if you are sure that you want to start internet sharing. Click Start.

In the "Share internet from" drop-down menu choose your internet source. Most of the time this is Ethernet.

in the "To Computers Using" menu choose AirPort.

Now you can connect to your router with other devices and enjoy internet over wifi.